We plan to investigate the effect of cross-links on DNA molecules containing single-strand breaks, and to measure the fraction of material that is reversibly bihelical. A theoretical relationship has been established which will be used as the basis for measuring the integrity of DNA molecules. Studies will also be conducted on the frequency with which interstrand cross-links and monoadducts are produced in phage DNA molecules following treatment with psoralen or khellin and 360 nm light. The action of intracellular enzymes on these damages will be investigated to determine which biochemical and genetic events can be attributed to interstrand cross-links and which to monoadducts. Gamma phages treated with psoralen and light will be used to investigate early steps in genetic recombination, particularly the interaction between the psoralen-treated phage DNA molecules and homologous duplexes in the same cell. BIBLIOGRAPHIC REFERENCES: Howard-Flanders, P. (1975) Repair by Genetic Recombination in Bacteria: Overview. In: Molecular Mechanisms for the Repair of DNA: Part A. (Eds. P. Hanawalt and R. B. Setlow), Plenum Publishing, New York. pgs. 65-76. Howard-Flanders, P., P- F. Lin and E. Bardwell (1975) Genetic Exchanges Induced by Structural Damage in Nonreplicating Phage DNA. In Molecular Mechanisms for the Repair of DNA: Part A. (Eds. P. Hanawalt and R. B. Setlow), Plenum Publishing, New York. pgs. 277-283.